1. Field of the Invention
The present invention relates to the art of earth boring and the collection of formation fluid samples from a wellbore. More particularly, the invention relates to methods and apparatus for collecting a deep well formation sample and preserving the in situ constituency of the sample upon surface retrieval. Once the sample is retrieved, this invention describes methods and apparatus for isolating and extracting a sub-sample for a field determination of the quality of the primary sample without altering the primary sample composition.
2. Description of Related Art
Earth formation fluids in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids control the phase relation of these constituents. In a subsurface formation, high well fluid pressures often contain gas within the oil above the bubble point pressure. When the pressure is reduced, as by raising an in situ captured sample of the formation fluid to the surface, the dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir.
Certain techniques analyze the well fluids downhole in the wellbore. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore.
Downhole temperatures in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at 70 degrees F., for example, the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop changes the in situ formation fluid parameters thereby inducing phase separation between liquids and dissolved gases within the formation fluid sample, for example. As another example, dramatic pressure changes in a formation sample may precipitate dissolved solids such as waxes and asphaltines. These types of phase separation represents significant and irreversible changes in the formation fluid characteristics, and reduces the ability to evaluate the actual properties of the formation fluid.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) teaches the concept of pressurizing a formation fluid sample with a hydraulically driven piston powered by a high pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) teaches the use of a pressurized gas to charge the formation fluid sample. U.S. Pat. Nos. 5,303,775 (1994) and U.S. Pat. No. 5,377,755 (1995) to Michaels et al. disclose a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling does not reduce the fluid pressure below the bubble point.
More recently, U.S. patent application Ser. No. 09/648,410 by Paul A. Reinhardt, filed Aug. 25, 2000, has disclosed a multiple tank sample extraction system in which each sample tank in a magazine carrier has a two stage piston chamber by which the in situ wellbore pressure of a deep well fluid within a sample retrieval chamber is amplified to overcome the contraction consequences of removing a sample of deepwell fluid to the earth surface. At the interface of the apparatus where each of several independently removable tanks is severed from a common charging magazine, a small quantity of high pressure formation fluid is isolated in a sample transfer conduit between a magazine distribution valve and a tank closure valve. Although both valves are closed when an individual tank is removed from its respective magazine alcove, this small quantity of fluid is vented to the atmosphere as a preparatory step to severance of the tank from the magazine for individual transport and sample testing.
Although the quantity of this atmospherically vented fluid is small, it is important to observe the nature and quality of the vented fluid as a qualitative clue to the fluid within the main body of the sample chamber. Notwithstanding extreme care in downhole sampling procedures, it is still possible for the wireline magazine to return with contaminated samples in one or more tanks. Such contamination may take the form, for example, of water seepage from other strata, mud cake deposited against the borehole wall or wellbore drilling fluid. Filtrate from oil based drilling mud is especially a problem.
Samples must be representative of fluid in the formation and consequently must be substantially free of contaminates from drilling operations. In particular, samples need to contain less than a few percent of filtrate from an oil base mud for that sample to be representative of the formation fluid. Usually, 10% contamination in a sample is too much for a reliable pressure/volume/temperature analysis. Acquisition of a formation fluid sample this pure and greater is difficult to obtain. Moreover, it is essential to know the relative contamination in a sample to a reasonable degree of certainty at the time the sample is extracted. The physical and intellectual effort committed to extracting a deepwell sample is of such magnitude that repetition of the effort is to be avoided if possible. Consequently, it is desirable to obtain a small sub-sample of the recovered fluids to determine whether or not the contamination level is sufficiently low to warrant laboratory analysis. It is imperative that this sub-sample be extracted without altering the physical properties of the primary sample reserved for a more expansive laboratory analysis.
It is an object of the present invention, therefore, to controllably secure a portion of the transfer conduit fluid for the purpose of field analysis. Also an object of the present invention is provision of means to evaluate the nature of a fluid sample confined within a high pressure tank chamber without risking the integrity of the sample composition.
These and other objects of the present invention as will become apparent from the following description of the preferred embodiments are accomplished by a deep well sampling system that is capable of isolating the last portion of sample fluid that is collected into a sample chamber. The sampling system extracts formation fluid directly from the desired formation through a probe that is pressed into the borehole sidewall. This formation fluid is pumped by a downhole equipment pump dedicated to the wellbore equipment along a pump discharge conduit and through a distribution valve or valves. The distribution valve is controlled to direct the flow of pumped fluid drawn from the borehole sidewall into a selected tank or into the wellbore. In a preferred embodiment, each of the tanks may be selectively separated from the magazine for reduced transport weight and handling bulk.
From the distribution valve, the pumped fluid flow is directed along respective supply/discharge conduits having at least two valves between the distribution valve and a respective sample receiving chamber. Significantly, the two valves are positioned along a respective supply/discharge conduit so that the conduit volume between the two valves is greater than the conduit volume between the distribution valve and the outermost of the two valves. Additionally, the conduit volume between the two valves should be about 1% to about 1.5% of the sample chamber volume or more.
A representative embodiment of the invention includes sample tanks having a compound piston within a tank housing interior. The compound piston defines the fluid sample chamber wherein the piston is moveable within the housing interior to selectively change the fluid sample chamber volume. The compound piston comprises an outer sleeve and an inner sleeve. The inner sleeve is moveable relative to the outer sleeve and both are moveable relative to the housing. However, movement of the inner sleeve relative to the outer sleeve is unidirectional. Both sleeves are displaced toward the lower head end by filling the sample collection chamber with formation fluid. A piston face portion of the outer sleeve includes a fluid transfer conduit that is flow controlled by a normally closed valve. The valve is opened by physical engagement with the lower head end of the tank housing. The lower head end of the housing includes a conduit that may be opened directly to the wellbore fluid via a valve in the magazine body that is controlled from the surface. Consequently, when the outer sleeve piston valve is opened by engagement with the lower housing head, wellbore fluid at wellbore pressure is admitted through the outer sleeve piston into an inner chamber. Wellbore pressure in the inner chamber displaces the inner sleeve relative to the outer sleeve whereby the solid structure of the inner sleeve cylinder edge is forced into the high pressure liquid sample chamber. Since the high pressure liquid sample chamber is a completely filled liquid volume, the inner sleeve cylinder edge penetrates the sample chamber only by compression of the liquid.
When the magazine and all tanks are returned to the surface, each tank is separated from the magazine for either shipment to an analysis laboratory or for immediate sample analysis. Because the fluid samples always contain some percentage of filtrate (contamination), it is important to assess the level of contamination without altering the sample volume within the sample chamber and before incurring the expense of laboratory analysis. Often, contamination of less than 10% is acceptable. Under certain conditions, however, a sample may have over 30% contamination and that is usually unacceptable.
For separation of a tank from the magazine, the outer conduit valve is closed. Supply/discharge conduit fluid between the distribution valve in the tank magazine and the outer conduit valve is vented through the wellbore fluid valve in the magazine. Various methods may be employed to investigate or retrieve the sub-sample for sample quality or contamination level. For example, a sight glass or optical port may be employed in the sub-sample conduit to visually or optically determine the sample quality. Other methods may include transfer of the sample into a controlled environment for analysis.
When the tank is free of the magazine, a low pressure receiver tank may be secured to the supply/discharge conduit nipple that serves as the connective interface between the tank and the magazine. With the tank supply/discharge conduit valve most proximate of the high pressure tank chamber closed, the outer valve is opened to release the formation fluid trapped between the two conduit valves into the low pressure receiver where it may be field examined. From such field examination, it may be determined whether the sample is excessively contaminated by wellbore water, oil, mud cake, drilling fluid or oil filtrate from oil based drilling mud.
After the field sample is extracted, the outer conduit valve is again closed and the tank disposed for completion of the laboratory analysis.